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参数资料
| 型号： | L5985TR |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 稳压器 |
| 英文描述： | 2 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 3 X 3 MM, 1.08 MM HEIGHT, LEAD FREE, VFQFPN-8 |
| 文件页数： | 7/37页 |
| 文件大小： | 856K |
| 代理商： | L5985TR |
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L5985
Application information
Doc ID 13006 Rev 5
15/37
5.2
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value in order to have the expected current ripple has to be selected.
The rule to fix the current ripple value is to have a ripple at 20%-40% of the output current. In
the continuos current mode (CCM), the inductance value can be calculated by the following
equation:
Equation 6
Where TON is the conduction time of the internal high side switch and TOFF is the conduction
time of the external diode (in CCM, FSW = 1 / (TON + TOFF)). The maximum current ripple, at
fixed Vout, is obtained at maximum TOFF that is at minimum duty cycle (see previous section
to calculate minimum duty). So fixing
ΔI
L = 20% to 40% of the maximum output current, the
minimum inductance value can be calculated:
Equation 7
where FSW is the switching frequency 1/(TON + TOFF).
For example for VOUT = 3.3 V, VIN = 12 V, IO = 2 A and FSW = 250 kHz the minimum
inductance value to have
ΔI
L = 30% of IO is about 18 μH.
The peak current through the inductor is given by:
Equation 8
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
In the table below some inductor part numbers are listed.
Table 7.
Inductors
Manufacturer
Series
Inductor value (
μH)
Saturation current (A)
Wurth
PD type S
3.3 to 6.8
2.75 to 4.2
Coilcraft
MSS1038
15 to 18
3.2 to 3.6
MSS7341
3.3 to 6.2
2.5 to 3.5
Coiltronics
CD1
15 to 22
2.9 to 3.6
UP2.8B
4.7 to 10
2.7 to 3.9
ΔI
L
V
IN
V
OUT
–
L
------------------------------ T
ON
V
OUT
V
F
+
L
---------------------------- T
OFF
==
L
MIN
V
OUT
V
F
+
ΔI
MAX
----------------------------
1D
MIN
–
F
SW
-----------------------
=
I
LPK
,
I
O
ΔI
L
2
--------
+
=
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